Barrier

ABSTRACT

The invention relates to a passive barrier ( 10 ) for mitigating blast waves an decelerating incident material to reduce the risk of sympathetic explosions a reduced stand-off distances. The barrier is configured to redirect the force of an incident blast wave and reduce the momentum of any incident material wherein the barrier is constructed of a shock absorbing foam material.

This invention relates to a barrier and more particularly to a barriersuitable for mitigating the effects of an explosion.

It is known that if an explosive device is detonated, it can detonateanother explosive device nearby through the effect of the blast wavegenerated, and/or by the impact of fragments of the exploded device. Ina situation where there are many explosives or explosive devices withina close proximity of one another, it is essential to protect as manyitems as possible from the risk of being detonated by a nearbyexplosion.

When an explosive device is detonated there are three principlemechanisms by which another device may be caused to detonate, termed asympathetic explosion. Firstly, the blast wave caused by the explosionof a first device can impact a nearby device with enough force todetonate it or at least damage it. However, the blast wave degradesquickly so the risk of devices being detonated falls off quickly withdistance. Secondly, the fragments of the exploded device and any otheritems nearby can become projectiles which radiate out from the device.These fragments, termed incident material, can impact a device withenough energy to detonate the device. The incident material can alsocause damage to the device which could render it unstable and vulnerableto detonation. Thirdly, the blast wave can lift and/or carry objects inits path which then themselves become projectiles which can damage ordetonate other devices.

An explosive device can be anything from a simple firework to a hi-techmilitary missile or bomb which contains and explosive material. Thedetonation of such an explosive device is caused by a shockwavepropagating through the explosive material contained within a device.The explosive force is released in a direction perpendicular to thesurface of the explosive which is often shaped to follow the internalcontours of the device.

Where the explosive device is a bomb, the bomb typically has a casingwhich has a first end and a second end distal from the first end. Thefirst end is a base plate and the second end is often shaped to improvethe aerodynamics of the bomb. The casing contains an explosive material.

Fragments from a 1000 lb (454.55 kg) bomb can be expected to travel sideon to a radius of 1.3 km and end on (from the base plate) to a radius of1.65 km in an unmitigated event, i.e. an explosion where no barrier isused. As can be expected, it is desirable to prevent the fragmentstravelling such a distance.

Previous trials for mitigation of such 1000 lb (454.55 kg) bombs haveshown that a second such explosive device, hereinafter termed anacceptor bomb, can be initiated by penetrating high velocity fragmentsfrom the detonated explosive device, hereinafter termed the donor bomb,from incident material or from the impact of the subsequently drivenmitigation barrier. So, it is necessary to slow down the incidentmaterial to a speed where if they do contact an explosive device it isnot detonated. A review of current fragment attack literature indicateda fragment strike velocity in the region of 600 ml/s (7.4 kJ energy) asthe threshold below which no sympathetic explosion was anticipated forany donor bomb. For experiments using a 1 mm thick aluminium shieldaround the explosives, the threshold appeared to rise to 800 m/s (13kJ). Thus for all subsequent research, the inventors have used theregion of 600-800 m/s as a target to which the barriers must retard thevelocity of the incident material.

It is known to use a barrier in a situation where explosives are storedor where there is the possibility that an explosion could occur. Suchbarriers may be reactive, for example, reactive armour, or passive,which do not have active components. Concrete has been employed in thepast to make a passive barrier to withstand the destructive force of anexplosion, such as the detonation of a bomb. However, barriers made fromconcrete take time to construct and once constructed are permanent. In aconflict situation, for example, it is required that explosive devices,such as missiles, are moved around and therefore the concrete barrierswhich have been built become disused and further barriers need to beconstructed elsewhere. It is evident that this practice requires muchtime and material. One solution to this problem has been to use waterfilled barriers. Water is well known in the art for mitigating blasts. Awater filled barrier may comprise one or more containers filled withwater which is/are placed between the explosive device(s) and theitem(s) to be protected. The water filled barrier overcomes the previousproblem as the barrier can be removed after use. However, the barrierneeds to be erected where there is an adequate water supply. In an areawhere a water supply is poor the water to fill the containers needs tobe transported. The barriers are often bulky which can posetransportation problems and add to the cost of using them. Also, the actof filling the containers with water will take time, as will emptyingthem. Once the water barrier is in place it will prevent shock waves andparticulates from detonating nearby explosives by acting as a shield.However, there is the possibility that the resulting blast and shockwaves can move this barrier and cause it to impact the item beingprotected which could result in physical damage to the item which inturn could cause it to detonate or become vulnerable to detonation.

It is recommended to have a minimum stand off distance of at least 1 mbetween the donor bomb and any barriers to ensure that weapon fragmentsstrike the barrier before the barrier is disrupted by the blast shock.This is the case for the composite barrier disclosed in patentapplication no. PCT/GB2006/004722. The barrier in this application wasinvented by the same inventors as the present barrier. It comprises aportable barrier which is capable of disintegrating harmlessly in thecourse of its effectiveness. This barrier is suitable for use where thestand-off is about 1 metre or more and has been proven in trials tomitigate against the effect of detonations of various militarymunitions. With this barrier the fragments of the bomb overtake theshockwave where the distance between the bomb and barrier is greaterthan or equal to 1 metre, so the barrier addresses the fragments beforethe shock wave disrupts the barrier. However, when the distance betweenthe barrier and the explosion is less than 1 metre, the barrier has todeal with the shock wave first which could, in some instances, begin todestroy or disrupt the barrier, thus rendering the item to be protectedvulnerable to impact by the blast wave and any incident material. Also,where the distance between the barrier and explosion is less than 1metre, the barrier itself can present an impact threat to the item to beprotected.

The distance between the bombs under a typical military fighter aircraftis less than 1 metre, and is typically only about 0.78 m so a differentbarrier needs to be used to protect these devices.

It is an object of the present invention to provide a barrier which issimple, cost effective, quick and easy to put in place and remove, whichis small, relatively light and portable and which provides adequateshielding from an explosion, even at small stand-off distances where theblast wave arrives either prior to or simultaneous with the incidentmaterial and which does not itself become a projectile capable ofdetonating an explosive device in the event of a blast.

Accordingly, the invention provides a passive barrier for mitigatingblast waves and decelerating incident material to reduce the risk ofsympathetic explosions at reduced stand-off distances, which isconfigured to redirect the force of an incident blast wave and reducethe momentum of any incident material wherein the barrier is constructedof a shock absorbing foam material.

The barrier according to the invention will fragment which means that asthe barrier moves under the force of the blast, it disintegrates andwill break up into smaller pieces which are not heavy enough to causedetonation of a nearby device as they do not have a mass great enough tocause damage if they impact a device. The barrier can be designed tofully arrest incident material if the sensitivity of the acceptor devicerequires. Otherwise, it is not always necessary for the barrier to fullyarrest the projectiles, but to retard the velocity of the projectiles tosuch an extent that they no longer present a sympathetic explosionthreat and therefore reduce the likelihood of a maximum credible event.

The barrier according to the present invention can, for example, be usedunder aircraft where the distance between the explosive devices is lessthan 1 metre. Due to the preferred material used to construct thebarrier, the barrier is able to disintegrate quickly and therefore canbe used where the distance between the barrier and the explosive deviceis less than 1 metre.

The barrier is advantageously constructed of a polyurethane foam. Moreadvantageously, the foam is a rigid polyurethane foam and is even moreadvantageously selected from the Last-a-Foam® FR3700 series produced byGeneral Plastics for packaging purposes which are closed cell rigidpolyurethane foams. The density of the foam chosen for a barrier isdependent upon the explosive charge it is to protect against.

The barrier can be any size depending upon its desired use. For example,it is advantageous to have a barrier which is the same size as or biggerthan the device to be protected or to be protected against so as toensure that as much of the device(s)/items are protected. Also, if thebarrier is bigger than the item to be protected then accurate placementof the barrier may not be so crucial. This will of course beadvantageous where there is little time to ensure accurate positioning,such as in a conflict situation or where a threat has only just beenidentified. However, the barrier can be smaller than the explosivedevice so long as it protects the assessed vulnerable area.

The shape of the barrier also depends upon its intended use. Forexample, in the preferred use the barrier is used to protect one bombfrom the other under a typical military fighter aircraft as it is notknown which, if either, of the bombs is likely to detonate. In thissituation the sides of the barrier which are most proximate to the bombsare configured to redirect the force of an incident blast wave. In asituation where an explosive device is to be placed so as to protect anon-explosive device or object, such as a vehicle or a person, it isonly necessary for the barrier to be configured to redirect the force ofan incident blast wave on the side proximate to the explosive device. Ineither of these embodiments, the barrier can be configured to redirectthe force of the blast wave by angling the sides proximate to the itemsto be protected. For example, if it is required to redirect the blastwave above and below the barrier to the same extent, then an apex isformed which aligns with the central line of the device to be protected.If the blast wave needs to be angled up or down relative to theexplosive device, then the side of the barrier proximate to the deviceslopes out or in respectively from the top of the barrier. The side ofthe barrier distal to the explosive device may be of any shape such asplanar, curved or angled. Shaping the barrier in this way ensures thatthe blast wave is deflected around the barrier so as to reduce the blastloading on it, thereby reducing the velocity by which the mass of thebarrier is driven at the item(s) to be protected and consequentlyreduces the likelihood of substantial damage to the acceptor munitionsat very low standoff distances.

The barrier is advantageously substantially diamond shape in the crosssection perpendicular to a first explosive device. The exterior of thebarrier forming the diamond cross section can be planar, concave, convexor any other configuration. Most advantageously, the barrier is shapedto have all four internal angles at about 90°. The diamond shape isthought to redirect the blast wave from the explosion around thebarrier. The loading of the blast wave is deflected from the apex andalong the barrier so that the pressure of the wave is spread over agreater area than would be the case with a flat surface. As much of theblast force is deflected there is less force left to move the barrierthan with a barrier having a flat surface. It is also believed, althoughthe inventors do not wish to be bound by theory, that the diamond shapecauses the fragments of the exploded device to move at differentvelocities and hence the fragments and barrier do not move together atthe same speed.

The barrier will be of particular use in situations where there arereduced standoff distances e.g. under a typical military fighteraircraft. The barrier may be suspended from the central pylon of theaircraft to locate it centrally between the two bombs. Alternatively,the barrier may be mounted or supported in a frame, holder or on wheelswhereby it can be manoeuvred into place. Preferably, the barrier ismounted or supported in a frame which is capable of being manoeuvredinto place quickly and easily. The frame may have height adjusting meansso that it may fit under an empty or laden aircraft such that thebarrier itself is aligned with the explosive device or item to beprotected. This may be achieved by spring loading the frame or byjacking or other such height adjusting means. The frame may furthercomprise braking or choking means for maintaining it in place. The framecan be made of any suitable material for use in a number ofenvironmental conditions. The barrier may also be used to protectweapons on different aircraft. Again, the barrier can be placed where itis needed.

In another embodiment, the barrier may have a casing to support it andto provide protection from minor impact damage. It is furtheradvantageous that the casing is water tight. The casing may enclose theentirety of the barrier or may partially surround the barrier to affordit some protection from being handled, the environment in which it isused and the weather. The casing may fit closely around the barrier orthere may be an airgap formed between the casing and the barrier, as isevident with the square configuration shown in FIG. 4. It has been shownthat if a small airgap exists between the barrier and the outer casingthere is no effect on the performance of the barrier. A preferredmaterial for the casing is polyethylene but any suitable material can beused. Further, it is advantageous that the polyethylene is about 10 mmthick. The barrier casing may include stiffening means which can supportthe barrier. The barrier with casing may be mounted or supported in aframe or holder as previously described.

Although the invention can be employed in close proximity to explosivesand explosive devices such as bombs and missiles, it is not limited tosuch use. The barrier can be placed between torpedoes in a submarine,between stationary vehicles carrying explosive devices or in civil ormilitary explosives stores. The barrier can also be used in a vehiclefor transporting and/or storing explosives. The barrier can further beused to surround a vehicle or vehicles to afford them increasedprotection. The barrier could also be used for packaging explosives orexplosive devices for transportation and storage. These uses are purelyfor illustration and do not restrict the scope of use of the invention.

The invention will now be described with reference to the accompanyingfigures:

FIG. 1 is an end on view of a barrier according to the present inventionsuspended from the central pylon of an aeroplane.

FIG. 2 is a plan view of the barrier positioned between two bombs.

FIG. 3 shows the barrier of the invention with a casing

FIG. 4 shows the barrier of the invention with a square casing

FIGS. 5 a-5 d show the barrier in different configurations for directingthe blast wave.

FIG. 1 shows the barrier 10 suspended from the underside of an aeroplane12 which is positioned between two bombs 14. The barrier is in theadvantageous diamond cross section configuration which will provideprotection to/from detonation of either bomb 14.

FIG. 2 is a plan view of the barrier 10 from above, which is positionedhalfway between two bombs 14. The barrier 10 is centred between the twobombs parallel with the side faces of the barrier 11 and aligned so thatthe noses 16 and bases 18 of the bombs are shielded from one another bythe barrier 10. The barrier is placed halfway between the two bombs soas to protect one from the other. The distance between the bombs (x) isonly 0.78 metres under a typical military fighter aircraft.

FIG. 3 shows the barrier 10 surrounded by a casing 20 which is shaped tofollow the contours of the barrier (shown in a broken line) closely. Thecasing is close fitting but has been shown with a small airgap forclarity of drawing.

FIG. 4 shows the barrier 10 with a square casing 22. There is an airgaparound the barrier due to the square shape of the casing 22 relative tothe diamond shape of the barrier 10.

FIG. 5 a shows a barrier 10′ having an angled side 13 proximate to anexplosive device 14′. The apex 24 of the angled side aligns with thecentre line 26 of the device 14′.

FIG. 5 b shows a barrier 10″ having a slope 28 which will direct theblast wave upwards, as shown by the dotted line. The barrier 10″ can beinverted so that the slope 28 will redirect the blast wave downwards.

FIGS. 5 c and 5 d show the exterior of the barrier 10 having concave andconvex configuration whilst still maintaining a substantially diamondshaped cross section.

Trials have proven the success of a barrier according to the presentinvention when used to protect against the detonation of a 1000 lb(454.55 kg) bomb. The trial was set up to replicate the use of thebarrier under a typical military fighter aircraft with the separationbetween the bombs being 0.78 m and having a barrier with a diamond crosssection midway between the two. One of the 1000 lb bombs was filled withconcrete to be an acceptor bomb so that the effects of stand-offdistance, barrier thickness, mass and the physical damage imparted tothe concrete filled bomb could be analysed and recorded after thecontrolled detonation of a live bomb to see how much damage was causedto the device. Polyurethane barriers with various nominal crushstrengths were tested as is shown in table 1.

TABLE 1 Polyurethane foam reference numbers, their corresponding nominalcrush strength and performance. Nominal Crush Foam number strength (MPa)Damage to acceptor bomb 3715 5 Slight distortion, fragment strikes 37187 Slight distortion, many fragment strikes 3725 12.5 Slight distortion,few fragment strikes 6725 13.5 Slight distortion, few fragment strikes3730 17.5 No significant distortion, few fragment scuff marks. Nodetonation when acceptor bomb was a live bomb. No barrier n/aSignificant fragment strikes and considerable distortion either of whichis enough to trigger a sympathetic explosion

All of the barriers tested greatly reduced the number and extent ofimpact marks on the acceptor bombs compared to not having a barrierpresent. When a 1000 lb bomb was detonated the concrete filled acceptorbomb the other side of the barrier to the explosion only received a fewscuff marks caused by fragments and was only slightly distorted. Thisamount of damage was deemed not to be enough to cause detonation of alive device. The 17.5 MPa crush strength foam showed the best result inthis trial as there was minimal damage to the acceptor bomb.

To prove this, the inventors used a live donor bomb, which was detonatedremotely, and a live acceptor bomb, each separated from the other by adistance of 0.78 m with a 17.5 MPa crush strength foam barrier placedbetween them. The barrier mitigated against a sympathetic detonation byretarding the velocity of the fragments and lessening the effects of theblast wave. Only a few fragment scuff marks were visible on the acceptorbomb casing and there was no significant distortion. It was evident thatthe damage was minimal and so the acceptor bomb was unreacted withminimum damage. After detonation of the bombs there were no remains ofthe barriers.

As mentioned previously, the crush strength of the rigid polyurethanefoam depends upon the size of the possible explosive event, the weaponfragment type and the distance between munitions. For mitigating againstthe sympathetic explosion of a 1000 lb (454.55 kg) bomb, the preferredcrush strength of the polyurethane foam is from 5 MPa to 17.5 MPa and isadvantageously in the range of from 12.5 MPa to 17.5 MPa.

The barrier used in the trials had dimensions of 350 mm×350 mm×1350 mmand weighed approximately 36 kg. However, the barrier can be shaped orsized to the required size which the skilled man would be able todetermine. The barrier could be made to the exact required dimensions,but this would mean that it is down to the operator to position it inthe exact location for maximum protection. In a conflict situation thisis not ideal as precious time could be lost. It is therefore envisagedthat the barriers used will be larger than the devices they areprotecting from or protecting against detonating.

1. A passive barrier for mitigating blast waves and deceleratingincident material to reduce the risk of sympathetic explosions atreduced stand-off distances, which is configured to redirect the forceof an incident blast wave and reduce the momentum of any incidentmaterial wherein the barrier is constructed of a shock absorbing foammaterial.
 2. A barrier according to claim 1 wherein the barrier is madeof a shock absorbing polyurethane foam material.
 3. A barrier accordingto claim 2 wherein the barrier is made of a rigid polyurethane foam. 4.A barrier according to claim 1 wherein the barrier is of a substantiallydiamond cross section.
 5. A barrier according to claim 1 wherein thecrush strength of the barrier is from 5 to 17.5 MPa.
 6. A barrieraccording to claim 5 wherein the preferred foam crush strength tomitigate against explosion of a 1000 lb (454.55 kg) bomb at a stand-offof under 1 metre is 17.5 MPa.
 7. A barrier according to claim 1 whereinthe barrier has a casing.
 8. A barrier according to claim 7 wherein thecasing extends around at least a portion of the barrier.
 9. A barrieraccording to claim 7 wherein the casing follows the shape of thebarrier.
 10. A barrier according to claim 7 wherein the casing iscuboid.
 11. A barrier according to claim 7 wherein the casing is made ofpolyethylene.
 12. A barrier according to claim 7 wherein the casingfurther comprises stiffening means.
 13. A barrier according to claim 1wherein the barrier is suspended from the central pylon of an aeroplane.14. A barrier according to claim 1 wherein the barrier is mounted orsupported in a frame or holder.
 15. A barrier according to claim 1wherein the barrier is retained in position by means of a foot, a holderor by suspension.
 16. A barrier according to claim 1 wherein the barrierfurther comprises wheels to aid positioning.
 17. A method of using abarrier according to claim 1 comprising positioning the barrier betweenan explosive device and an item to be protected.